1. Field of the Invention
This invention relates to a high temperature steam turbine and more particularly to a method for automatically determining the temperature of the steam turbine rotor to permit loading of the turbine once its temperature reaches the transition temperature of the metal of the rotor.
2. Description of the Prior Art
The procedure for starting a high temperature steam turbine generally requires an initial heating of the rotor and cylinders to provide uniform build-up of heat for uniform expansion, thereby reducing thermally induced stresses and also requires the rotor to be heated above a temperature, referred to as the transition temperature, wherein the metal changes from a brittle character to a ductile character to avoid the hazard of brittle fracture caused by prematurely loading or stressing the rotor at temperatures below the transition temperature.
Heretofore, it has been common practice to provide a heat soak during startup wherein steam from the primary boiler was introduced into the cylinders through the throttle valves to bring the rotor up to about two-thirds normal speed and heat the rotors to at least 250.degree. F., (generally considered to be the transition temperature) prior to bringing the rotor up to full speed and loading it. This generally requires on the order of 8 to 9 hours to fire the boiler plus an additional 4 to 6 hours based on heat transfer calculations to attain the 250.degree. F. temperature of the rotor. In some instances, thermocouples are placed on a stationary structure generally adjacent the rotor to indicate the temperature of the stationary structure as a guide to the rotor temperature. This heating period for the turbine rotors is commonly referred to as a running heat soak.
In an attempt to reduce the time required to heat the rotor to the desired temperature and also reduce the energy required to provide this heat, a prewarm cycle has been recently utilized wherein steam from available auxiliary boilers, generally having much less capacity and therefore more quickly elevated to a high temperature, was introduced into the cylinders of a high temperature turbine while the main boiler was being fired. Also, in instances where the main boiler was ready prior to the turbine rotor temperature being above the transition temperature, steam from the main boiler would also be introduced to expedite the process in a normal running heat soak. However, because of variations in temperatures and capacities, the heat soak period could not be standardized for all applications. Although thermocouples could be placed on the stationary structure adjacent the rotor, there was sufficient discrepancy between this temperature indication and the actual temperature of the rotor that an alternate, more accurate system of determining the rotor temperature would be useful to assure the operator that the rotor was at least above the transition temperature and the turbine available for loading.
One method of determining the temperature of the rotor comprises manually measuring certain axial dimensional changes of the rotor and cylinder and determining the axial expansion therefrom which in turn permits calculation of the temperature of the rotor. Although in the abstract, it would seem that such manual measurements could readily be made, in practice, because of the limited space available for such measurements, the complex nature of the structure involved in addition to the rotation thereof, and the generally hot environment in which such manual measurements were required to be made with extreme accuracy, the measurements were awkward and inconvenient to manually acquire. Therefore, the process was apt to be ignored. Further, each time such measurements were taken, (i.e. at regular short term intervals during warmup) it was necessary to insert them into a certain summation prior to obtaining the total axial dimensional change, and this result was again required to be used either through formulas, or graphs, etc., to determine the temperature of the rotor.